Messages in this thread | | | Date | Tue, 28 Jun 2022 11:04:59 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH] vfio: remove useless judgement |
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On Tue, Jun 28, 2022 at 09:54:19AM -0400, Steven Sistare wrote:
> >> As you and I have discussed, the count is also wrong in the direct > >> exec model, because exec clears mm->locked_vm. > > > > Really? Yikes, I thought exec would generate a new mm? > > Yes, exec creates a new mm with locked_vm = 0. The old locked_vm count is dropped > on the floor. The existing dma points to the same task, but task->mm has changed, > and dma->task->mm->locked_vm is 0. An unmap ioctl drives it > negative.
Oh.. This is probably a bug, vfio should never use task->mm, the mm itself should be held using mmgrab instead.
Otherwise exec case is broken as you describe.
> I have prototyped a few possible fixes. One changes vfio to use user->locked_vm. > Another changes to mm->pinned_vm and preserves it during exec. A third preserves > mm->locked_vm across exec, but that is not practical, because mm->locked_vm mixes > vfio pins and mlocks. The mlock component must be cleared during exec, and we don't > have a separate count for it.
Lossing locked_vm on exec/fork is the correct and expected behavior for the core kernel code, the bug is that vfio drives it negative.
Jason
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